An elemental atom has electrons that reside in layered shells further away from the atomic nuclei. The farther the levels, the lesser the attractive force between nuclei and the electrons.
Silicon being an element that has four extra electrons in its outermost shell. These electrons can be supplemented with external energy enough to exit the atomic structure. When an electron exits, there is an empty space for other electrons to fill the exit hole.
When the electron exits, we say that the electron has exited the Valence Band (the uppermost shell which houses the valence electrons) into the Conduction Band (electrons in conduction band leads to the material becoming a conductor). This phenomenon generates multiple electron-hole pairs.
The energy required the excite the valance electrons into the conduction band is called the bandgap equivalent energy, and is measured in electron volts (eV).
The insulators have a larger bandgap requiring high energy inputs to bring their electrons to the conduction band. The metals have an overlap, meaning any free electrons will be conducted through the material without an option to control the flow. The semiconductors act like a switch because they do not freely conduct, but on input of energy equivalent to the bandgap, they start acting like conductors. Hence the use case of semiconductors as diodes.
Some bandgap equivalent energies for semiconductor materials,
| Material | Bandgap (eV) |
|---|---|
| Germanium | 0.67 |
| Silicon | 1.1 |
| Gallium Arsenide | 1.42 |
| Cadmium Telluride | 1.45 |
| Perovskite | 1.55 |